In foraging for food, animals face the problem of acquiring an appropriate selection of nutrients and sufficient energy in a minimum amount of time and with a minimum expenditure of energy. This problem could, in effect, be viewed as a cost-benefit analysis or optimization function. Among factors involved in its solution are foraging costs, consummatory costs, digestive costs, absorptive efficiency, efficiency of utilization, realized portion of growth potential, body weight maintenance, energy reserves, and reproductive potential. The biochemical bases of this problem are relatively unexplored. Traditionally, animal intake has been viewed as producing some optimal set of values for certain biochemical-physiological parameters (e.g., levels of blood glucose, fat reserves, size, etc.). However, for a given animal there may not be a single optimal state but, rather, a set of "optimal" states, any of which might be achieved as a result of different but acceptable solutions to the cost-benefit analysis. From this view, the animal might make ""tradeoffs" between a particular biochemical-physiological state (the consequence of this intake, or his "benefits") and the costs of attaining and maintaining that state. In consequence, animals self-selecting their diets might exhibit different patterns of intake depending upon the effort required to obtain and consume different dietary components, their palatability and caloric density, etc. The present research explores the manner in which animals equate the cost of selecting and procuring foods with (1) nutrient and energy values of those foods; (2) the animal's energetic expenditures; and (3) the resulting growth rate, size, composition, and health (reproductive potential) of the animal. BIBLIOGRAPHIC REFERENCES: Collier, G., Kanarek, R., Hirsch, E. and Marwine, A. Environmental determinants of feeding behavior: Or how to turn a rat into a tiger. In: Psychological Research: The Inside Story, M.H. Siegel and H. P. Seigler, eds., Harper and Row, New York. 1975. Collier, G. and Hirsch, E. Nutrient factors as determinants of sucrose ingestion. In: Taste and Development: The Genesis (Text Truncated - Exceeds Capacity)